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PubMedDate not supplied

Measurement of the replacement correction factor for parallel-plate chambers in electron fields.

When parallel-plate chambers are used for dosimetry in electron fields, the AAPM dosimetry protocol recommends a value of 1.0 for the replacement correction factor, P(repl),pp,E, until further data become available. Here, P(repl),pp,E for five commercially available parallel-plate chambers was measured as a function of electron energy from a nominal value of 5.5 to 22 MeV by comparison with a cylindrical chamber whose P(repl),cyl,E was obtained from data in the protocol. Since this method is based on the concept of a constant value for Ngas,pp, the energy and modality dependence of Ngap,pp is also investigated for these chambers for Co-60, 4-, 6-, 24-MV photons and for 22-MeV electrons. It is found that for three of the chambers P(repl),pp,E is independent of energy, consistent with unity within one or two standard deviations (s.d.). For the fourth chamber P(repl),pp,E is similarly consistent with one above 10 MeV, but decreases at lower energies, while for the fifth one it shows a systematic drop with decreasing energy.

Calibration↗

PubMed1999

A new radiographic procedure for obtaining correction factors of 3H-beta-self-absorption for quantitative tritium autoradiography.

A 3H-radiographic method based on the absorption of 3H-beta-particles by an overlying tissue section was established for obtaining correction factors of 3H-beta-absorption (c.f.s) on the cellular level for all kinds of sections in a simple and more precise way as possible with interferometry. Unlabelled paraffin or Araldite sections were mounted on a thin uniformly 3H-labelled section of resin, and autoradiographs were prepared. Grain densities of neuronal cell types and cell-free areas within and outside the paraffin or Araldite sections were evaluated in autoradiographs, where the exposure time or the thickness of the overlying histological section was varied. From these values c.f.s were calculated applying the Beer-Lambert law. It was shown that corresponding c.f.s determined with this new radiographic method correspond well with each other. However, they will only agree with those c.f.s obtained by interferometry, if relative c.f.s are compared. Since the c.f.s are quite sensitive to the section thickness, a new parameter phi was introduced, which helps to assess whether the microtome used works exactly. Generally, the method presented can be used on the cytological level as well as for whole areas in every other autoradiographic study.

Animals↗

PubMed1986

Denominator definition by the utilization correction factor method.

Most clinical research in the United States has been carried out in atypical populations. This study, done in a network of primary care populations in Virginia, calculates the denominator by the utilization correction factor method and compares the demography with that of the population of the state. The demographic characteristics of the patients in the network are very similar to those of the underlying populations and on the same order of state-of-the-art sampling methods in current use.

Adolescent↗

PubMed2006

Consideration on calibration and correction factors of an Hp10 chamber for different radiation qualities and angles of incidence.

This paper presents the characterisation performed at IRSN (France) of an H(p)(10) chamber in terms of calibration coefficient and correction factors for the radiation qualities of ISO narrow spectrum series. The chamber response, expressed in H(p)(10) using conversion coefficients h(p)(K)(10; N, alpha) listed in ISO 4037-3 in the energy range from 30 to 1250 keV and for angles of incidence between 0 and 70 degrees, was found to be within approximately 10%. However, for photon energies <30 keV, an overresponse of the chamber that could reach 100% was observed. Nevertheless, this overresponse was reduced to 25% using the conversion coefficients estimated at Physikalisch-Technische Bundesanstalt (PTB). This implies that the X-ray spectra produced by the IRSN X-ray units are very similar to those produced by PTB, both containing a little bit more high-energy photons than the spectra used in ISO 4037-3. The dose rate dependence of the chamber tested by gamma radiation from (60)Co sources was found to be within 2% in the range of 0.3 mSv h(-1) to 1 Sv h(-1). The H(p)(10) chamber can measure directly the conventional true value of H(p)(10) after calibration by a reference laboratory, and can be used for transferring H(p)(10) reference quantities from a reference laboratory.

Calibration↗

PubMed1985

Production of element correction factors for thermoluminescent dosimeters.

Approximately 80 processors of personal dosimetry in the United States use thermoluminescent dosimeters (TLDs). Recent demands that dosimetry processors be able to measure radiation doses to within +/- 50% of the correct value have focused attention on the reproducibility of the TL elements within each TLD. The phosphors for these TLDs are manufactured by three companies. A dosimetry processor faces three options concerning the quality of the TL elements purchased; trust the supplier's quality control program, screen new TL elements and discard those that are extremely bad, or use element correction factors (ECFs). The first option results in dosimetry processors failing the +/- 50% accuracy requirement due to excessive variability among the TL elements. The second option still permits large precision errors that come close to the +/- 50% accuracy requirement. This paper advocates the third option and presents a 10-step procedure to produce ECFs. The procedure ensures that the ECFs represent only variations among the TL elements and not variations caused by stability problems with the TLD reader. Following is an example of ECF production for 3000 TLDs.

Quality Control↗

PubMedDate not supplied

Comparison of calculated and measured heterogeneity correction factors for 125I, 137Cs, and 192Ir brachytherapy sources near localized heterogeneities.

The influence of tissue and applicator heterogeneities on brachytherapy dose distributions is not well understood, despite widespread use of shielded applicators in intracavitary therapy. Heterogeneity correction factors (HCF) have been measured using a silicon diode detector arising from bounded heterogeneities consisting of lead, steel, titanium, silver, aluminum, and air cylinders near brachytherapy sources of 125I, 137Cs, and 192Ir. In addition, transverse-axis dose distributions for the three sources in homogeneous water were measured for distances of 0.2 to 16.0 cm. For each point of measurement, relative diode readings were simulated by a Monte Carlo photon transport code utilizing accurate models of the source internal structure, the experimental measure geometry and the source-strength calibration geometry. Comparison of measured and calculated HCF's reveals excellent agreement (1%-3% average) over a wide range of materials, diameters, and thicknesses. In addition, Monte Carlo simulation not only accurately reproduced the relative transverse-axis dose distributions in homogeneous medium, but was able to predict the variation of diode response with photon energy with an accuracy of 3% over the range of 30-662 keV. Our measurements demonstrate that HCF's vary by as much as 60%-100% with distance and heterogeneity diameter for a fixed thickness. Finally, silicon diode measurements of HCF (denied as reading with heterogeneity/reading in homogeneous medium) is shown to lead to errors of 5%-30% for 137Cs and 192Ir sources in the presence of high-atomic number shielding materials. This paper concludes, that Monte Carlo simulation is a powerful, convenient and accurate tool for investigating the long-neglected area of brachytherapy heterogeneity corrections.

Biometry↗

PubMed2002

Electron beam therapy at extended SSDs: an analysis of output correction factors for a Mitsubishi linear accelerator.

The effects of extended source-to-surface distance (SSD) on the electron beam dose profiles were evaluated for various electron beam energies--6, 9, 12, 15 and 20 MeV-and the accuracy of various output correction methods was analysed on a Mitsubishi linear accelerator using a radiation field analyser (RFA). The dose fall-off region of the central axis depth-dose curves was nearly independent for SSDs up to 120 cm where as in the build-up region, a marginal reduction of surface dose was observed, particularly for lower energies and for smaller field sizes. Effective SSDs and virtual source distances were evaluated for field sizes ranging from 5 x 5 to 15 x 15 cm2 for various energies. Curve fitting was done with the measured outputs with various equations and coefficients were evaluated. The accuracy of the derived output correction factors by effective SSD, virtual source distance and curve-fit methods was assessed by evaluating correlation coefficients between the calculated and the measured values. The correlation coefficient was best with the linear-quadratic equation followed by the effective SSD method and the virtual source method. The output correction based on the linear-quadratic equation showed the best estimate of electron beam output at extended SSDs with an accuracy well within +/- 1%. The rapid reduction of dose due to the applicator-scattered component at d(max) point with an extended SSD was significant for the 5 x 5 cm2 applicator and lower energies.

Algorithms↗

PubMedDate not supplied

Monte Carlo calculation of the wall correction factors for ionization chambers and Aeq for 60Co gamma rays.

The application of cavity-ionization chambers to the standadization of 60Co gamma-ray beams, in terms of exposure, requires that the specific ionization of air Jg, be corrected for the attenuation and scatter of the incident rays by the wall, central electrode, and supporting stem of the chamber. A Monte Carlo photon-electron transport code has been developed for the purpose of calculating this correction for spherical and cylindrical chambers. The code has been applied to a spherical graphite chamber having dimensions typical of the chambers used by the NBS, the calculated wall-correction factor is in close agreement with the average of the NBS factors which were determined experimentally. The code was also used to calculate Aeq, which is central to the determination of tissue-air ratios. The calculated value, 0.989 +/- 0.003, is very close to the generally accepted value, 0.985.

Cobalt Radioisotopes↗

PubMedDate not supplied

The role of humidity and other correction factors in the AAPM TG-21 dosimetry protocol.

A detailed derivation is presented of the formulas required to determine Ngas and Dmed in the AAPM TG-21 dosimetry protocol. This protocol specifies how to determine the absorbed dose in an electron or photon beam when using exposure or absorbed dose calibrated ion chambers. It is shown that the expression given in TG-21's recent letter of clarification is incorrect. Accounting for humidity correctly increases, by 0.4%, all absorbed dose determinations using an exposure calibrated ion chamber. Taking into account other correction factors in the equation for exposure could also have varying, but significant effects (possibly over 1%). These are the stem scatter correction, the axial nonuniformity correction and the electrode correction for electrodes made of different materials from the wall. Attention is drawn to differences in the definitions of the exposure and absorbed dose calibration factors, Nx and ND, respectively, as supplied by the NBS and the NRCC.

Humans↗

PubMed1996

Ion-recombination correction factor ksat for spherical ion chambers irradiated by continuous photon beams.

The large range of reference air kerma rates of brachytherapy sources involves the use of large-volume ionization chambers. When such ionization chambers are used the ion-recombination correction factor ksat has to be determined. In this paper three spherical ion chambers with volumes ranging from 30 to 10(4) cm3 have been irradiated by photons of a 192Ir source to determine the ksat factors. The ionization currents of the ion chambers as a function of the applied voltage and the air kerma rate have been analysed to determine the contribution of the initial and general ion recombination. The ksat values for large-volume ionization chambers obtained by considering the general ion recombination as predominant (Almond's approach) are in disagreement with the results obtained using methods that consider both initial and general ion-recombination contributions (Niatel's approach). Such disagreement can reach 0.7% when high currents are measured for a high-activity source calibration in terms of reference air kerma rate. In this study a new 'two-voltage' method, independent of the voltage ratio given by a dosimetry system, is proposed for practical dosimetry of continuous x- and gamma-radiation beams. In the case where the Almond approach is utilized, the voltage ratio V1/V2 should be less than 2 instead of Almond's limit of V1/V2 < 5.

Biometry↗

PubMed1997

Correction factors for parallel-plate chambers used in plastic phantoms in electron dosimetry.

In electron beam dosimetry using parallel-plate chambers solid phantoms are sometimes necessary. To obtain the dose to water from the ionization obtained in the solid phantom, fluence correction factors and perturbation factors have to be applied. In this study fluence factors in a perturbation free geometry have been determined experimentally for common phantom materials. Wall perturbation factors for simulated Attix, NACP, and Roos chambers have also been determined for the same materials. Comparative Monte Carlo calculations have been performed using the EGS4 Monte Carlo code. Comparison with data in newly published protocols such as IAEA and IPEMB shows an agreement with the results obtained in this paper to within 1%, demonstrating that the data published in these protocols may be used with reasonable accuracy if recommended phantoms are used. The results also show that if unsuitable phantom materials are used, the wall perturbation factors may differ for different chambers and for different phantom materials by more than 3% and perturbation factors have to be considered in order to obtain a high accuracy in the dose determination.

Dose-Response Relationship, Radiation↗

PubMed2007

Impact of instrumentation on DaTSCAN imaging: how feasible is the concept of cross-systems correction factor?

AIM: The aim of this investigation was to study the impact of applying different camera emission computed tomography (ECT) systems on semi-quantitative analysis of the novel dopamine transporter (DaT) radioligand [123I]-ioflupane (DaTSCAN) and whether applying a system-dependent correction factor is feasible to allow a migration of a ''normal'' reference data set between systems. METHODS: Two triple-head systems, PrismXP-3000 equipped with high resolution Fan-Beam (PRISM) and Irix (Philips Medical Systems, Europe) equipped with high resolution parallel whole collimators (IRIX), were compared in phantom and patient studies. Acquisition and quantification parameters were standardized. The anthropometric striatal calibration phantom (RSD Inc., CA, USA) at rising striatum: background ratios between 2-11 was studied. Fifty-one consecutive patients (25 females, age 64.5+/-11.9 years, and 26 males, age 60.6+/-12.7 years) were randomized to both cameras for acquisition time typically beginning 3.5 to 4.5 h after i.v. injection of 123I-Joflupane at 2.06+/-0.47 MBq/kg. Striatal uptake of the radioligand was categorized as normal or abnormal, and abnormal images were further subdivided into 3 severity levels. Striatum: cerebellum ratio (SCr) was determined by applying a fixed circular or rectangular striatal and fuzzy cerebellar regions. The effect of introducing scatter compensation (Sc) on quantification and on SCr was investigated. RESULTS: The regression coefficient (Rc) of SCr computed for both systems was close to identity (IRIX=0.999x (PRISM)+0.48 and 1.01x(PRISM)+0.86 for right and left striatum). Rc of SCr values in the phantom studies was closer to identity when Sc was added (IRIX=0.982x [PRISM]+0.724 with Sc vs 1.22x[PRISM]-0.32 without Sc). SCr values were higher for IRIX by 5-10%. Including Sc increased the percent recovery and the linear dynamic range for both systems; however, fan beam recovery decreased at low SCr values. The delineation of the striatal boundaries improved after applying Sc. Identifying the cerebellar region was easier with PRISM owing to significantly higher count statistics. Both systems performed equally well with respect to image quality. Visual scoring by 2 observers correlated significantly on the k-test (K: 0.883, T: 10, P<0.0001). CONCLUSION: A simple linear correction is applicable to ''normal'' reference data set to migrate from one system to another; however, data acquisition and quantification parameters need to be standardized. Higher recovery values are dependent on system resolution. The spatial distribution and image quality of DaTSCAN on different high-resolution systems applying standardized acquisition and reconstruction protocols is less operator dependent and does not affect visual rating of striatal DaT loss.

Adolescent↗

PubMed1998

SCL 90-R interpretation and brain tumour: a correction factor?

OBJECTIVE: Determine whether spurious SCL 90-R profiles resulting from endorsement of neurologic treatment items can be corrected and still retain ecological validity. DESIGN: The proportion of subjects' item endorsement was compared with the adult non-patient norm group. Items with discriminative power (chi 2 > 25, p < 0.001 and endorsement by at least 25% of subjects) underwent principal-components analysis with somatic treatment and psychiatric factors identified. Items in the somatic treatment factor were extracted as the corrective factor. The resultant corrected T scores were plotted against original SCL 90-R profiles. SETTING: University medical centre. PATIENTS: Thirty community dwelling adults with biopsy confirmed malignant brain tumours referred by their treating physician for evaluation prior to aggressive treatment. INTERVENTIONS: The SCL 90-R was part of a neuropsychological test battery. MAIN OUTCOME MEASURE: The SCL 90-R was selected prior to data collection because it is the second most frequently used psychosocial instrument in medical settings, yet has demonstrated attenuated validity with neurologic patients. RESULTS: Extraction of 10 somatic treatment items resulted in lower profiles on Somatization, OBSESSIVE: Compulsive, Depression, and Anxiety dimensions and the global indicators. However, clinical caseness for individuals remained greater than indicated by clinical interview. CONCLUSION: Correction for SCL 90-R neurologic treatment items resulted in reduced sensitivity and poorer ecological validity.

Adult↗

PubMed1995

Calculation of sensitivity correction factors for surface coil MRS.

Quantification of MRS signals obtained with surface coils is difficult due to the inhomogeneous response of these coils. This inhomogeneity results in the measured signal from a defined volume of interest (VOI) being spatially dependent. To account for the sensitivity variation with position from the surface coil, we have developed a method of calculating correction factors for defined VOIs based on an experimentally obtained 3D sensitivity coil map. These factors may then be applied to spectra obtained from these VOIs to accurately take into consideration the varying coil sensitivity resulting in a reduction of measured signal. This method is demonstrated here to be able to correct for the inhomogeneity of surface coils over a range of two coil radii to within 4% accuracy.

Humans↗

PubMed2001

QSAR modeling of bioconcentration factors in fish based on fragment constants and structural correction factors.

The quantitative structure-activity relationship between the BCF and fragment constant of organic chemicals was studied using a database containing 337 experimental BCF values. The database covered a large variety of chemicals ranging from the very hydrophobic to the very hydrophilic with logKow values between 0.39 and 8.60. The structural features affecting the BCFs were identified and evaluated during a preliminary modeling. A final linear multivariate regression model was derived that was able to account for as much as 98.0% of the variation in the experimental BCF values. The mean absolute error for the final model was 0.315 log-units. In addition, the predictability and robustness of the model was also evaluated.

Animals↗

PubMed2005

Efficiency for unretained solutes in packed column supercritical fluid chromatography. I. Theory for isothermal conditions and correction factors for carbon dioxide.

A general theory for efficiency of nonuniform columns with compressible mobile phase fluids is applied to the elution of an unretained solute in packed-column supercritical fluid chromatography (pSFC). The theoretical apparent plate height under isothermal conditions is given by the Knox equation multiplied by a compressibility correction factor f1, which is equal to the ratio of the temporal-to-spatial average densities of the mobile phase. If isothermal conditions are maintained, large pressure drops in pSFC should not result in excessive efficiency losses for elution of unretained solutes.

Chromatography, Liquid↗

PubMed1999

Effect of electron contamination on scatter correction factors for photon beam dosimetry.

Physical quantities for use in megavoltage photon beam dose calculations which are defined at the depth of maximum absorbed dose are sensitive to electron contamination and are difficult to measure and to calculate. Recently, formalisms have therefore been presented to assess the dose using collimator and phantom scatter correction factors, Sc and Sp, defined at a reference depth of 10 cm. The data can be obtained from measurements at that depth in a miniphantom and in a full scatter phantom. Equations are presented that show the relation between these quantities and corresponding quantities obtained from measurements at the depth of the dose maximum. It is shown that conversion of Sc and Sp determined at a 10 cm depth to quantities defined at the dose maximum such as (normalized) peak scatter factor, (normalized) tissue-air ratio, and vice versa is not possible without quantitative knowledge of the electron contamination. The difference in Sc at dmax resulting from this electron contamination compared with Sc values obtained at a depth of 10 cm in a miniphantom has been determined as a multiplication factor, Scel, for a number of photon beams of different accelerator types. It is shown that Scel may vary up to 5%. Because in the new formalisms output factors are defined at a reference depth of 10 cm, they do not require Scel data. The use of Sc and Sp values, defined at a 10 cm depth, combined with relative depth-dose data or tissue-phantom ratios is therefore recommended. For a transition period the use of the equations provided in this article and Scel data might be required, for instance, if treatment planning systems apply Sc data normalized at d(max).

Electrons↗

PubMed1982

Development of a correction factor for Xe-133 vials for use with a dose calibrator.

Manufacturers of dose calibrators who gave calibration settings for various radionuclides sometimes do not specify the type of radionuclide container the calibration is for. The container, moreover, may not be of the same type as those a user might purchase. When these factors are not considered, the activity administered to the patient may be significantly different from that intended. An experiment is described in which calibration factors are determined for measurement of Xe-133 activity in vials in a dose calibrator. This was accomplished by transferring the Xe-133 from the commercial vials to standard NBS calibration ampuls. Based on ten such transfers, the resulting correction factor for the dose calibrator was 1.22.

Drug Labeling↗